VAPOR PRESSURE OF LPG (EXPANSION METHOD): ASTM D6897
EXPLANATION
Information on the vapor pressure of LPG is pertinent to selection of properly designed storage vessels, shipping containers, and customer utilization equipment to ensure safe handling of these products. It is important for safety reasons that the maximum operating design pressure of storage, handling, and fuel systems will not be exceeded under normal operating temperature conditions. For LPG, vapor pressure can be considered a semi-quantitative measure of the amount of the most volatile material present in the product. This test method uses a small sample volume and excludes any manual handling of a measuring chamber under high pressure. It covers product temperatures of 37.8° C, vapor to liquid ratio of 0.5:1, and pressures from 200 - 1550 kPa on a sample volume of 3.33 mL. This test method is applicable to the determination of vapor pressures of LPG products at temperatures from 37.8 - 70° C, vapor to liquid ratios of 0.1:1 to 4:1 and pressures up to 3500 kPa. This test method is not intended to determine the true vapor pressure of LPG samples, but rather determine the vapor pressure at the conditions mentioned above, just like the Test Method ASTM D1267. Thus, this test method will not measure the full contribution from any dissolved gases such as nitrogen or helium if they are present. The contribution of light gases to the measured vapor pressure is highly dependent on the test temperature, type of gas, and V/L ratio of the test.
TEST SUMMARY
Employing a measuring chamber with a built-in piston, the chamber is rinsed three times with a portion of the sample, which is then discarded. A sample of defined volume is drawn from a pressurized sampling system into the temperature controlled chamber at 5 ° C by moving the piston into the filling position. After sealing the chamber, the volume is expanded by moving the piston until the final volume produces the desired vapor to liquid ratio of 0.5:1. The temperature of the measuring chamber is then regulated to the test temperature of interest, such as 37.8° C. The observed total pressure at equilibrium is corrected relative to 101.3 kPa and reported as the LPG vapor pressure at the selected test temperature.
TEST PRECISION
Repeatability: 7.4 kPa (1.1 psi)
Reproducibility: Not known at present
Relative bias against Test Method ASTM D1267 is not yet known.
